Underground storage tanks are used to store hazardous substances and petroleum products. It is estimated that a significant proportion of the nearly five million tanks in the United States are leaking harmful products into the environment. To ameliorate this problem, the Environmental Protection Agency (the "EPA") has recently promulgated regulations which require that any leakage exceeding a rate of 0.05 gallons per hour be detected and contained.
Methods for detecting leaks in underground storage tanks are well known in the prior art. Most of these techniques use a quantitative approach to identify a leak or to determine leak rate based on a measurement of volumetric changes of the stored product in the tank. The capability of prior art leak detection methods to accurately measure leakage is affected by certain variables such as temperature change, tank deformation, product evaporation, tank geometry and the characteristics of the stored product. The most significant of these factors is temperature variation, which causes dynamic expansion or contraction of the stored product on both a short-term and long term basis. Indeed, changes in ambient temperature throughout the day are often large enough so as to "mask" the leakage rate to be measured. For example, a change of 0.01.degree. F. per hour in a 10,000 gallon tank will cause a 0.068 gallon change in the product volume per hour, thus offsetting or amplifying an observed leak rate.
Most of the prior art methods for leak detection attempt to compensate for such temperature variations. In quantitative techniques, i.e., tests based on product volume changes, temperature in the tank is typically sensed by a plurality of temperature sensors located at various levels or stratifications therein. The sensed temperature data is collected and processed to measure the volumetric average of product temperature during a test. Other techniques attempt to compensate for temperature variations by performing the leak test over very short or long time intervals. All such techniques are unsuccessful because of the difference between the measured temperature change and the actual temperature change during the applicable test interval.
One solution to the problem of temperature-induced volumetric changes in underground storage tank leak detection is described in U.S. Pat. No. 4,732,035 to Lagergren et al. This patent describes an apparatus comprising an elongated pressure tube having first and second ends an inlet adjacent the first end, and a substantially hollow core for supporting a liquid having a volumetric coefficient of expansion per degree Fahrenheit or Centigrade (a so called "temperature coefficient") substantially lower than the temperature coefficient of the stored product A bladder, formed of a material which expands or contracts with substantially no resistance to flow of the low temperature coefficient liquid, is connected to the pressure tube adjacent the second end thereof. The bladder supports a first portion of the low temperature coefficient liquid in static equilibrium with respect to a second portion of the liquid supported in the tube. The large disparity between the temperature coefficients of the liquid and the stored product insures that temperature-induced volumetric changes in the stored product do not cause material variations in the level of the liquid in the pressure tube. Accordingly, any fluctuation in the liquid level in the tube represents a true indication of leakage of the fluid product out of the storage tank or leakage of a foreign product (e.g., ground water) into the storage tank.
The apparatus described in U.S. Pat. No. 4,732,035 substantially eliminates measuring inaccuracies in a storage tank leak detection system. Given the benefits of this technology, it would be desirable to provide additional improvements thereto to further limit the potential for any measuring inaccuracies and to provide reliable and easy to use testing equipment.